1. Field of the Invention
The present invention relates to an input device of a computer, and more particularly, to an input device for transmitting variable input signals to a computer game port.
2. Description of the Prior Art
Many computer input devices, such as a joystick, import their input signals to a computer through its game port. A personal computer game port comprises a capacitor and a detecting circuit for detecting a time period needed for charging the capacitor from 0 V to a threshold voltage. The detected time period will be transmitted as an input signal to the computer and the capacitor will be discharged for another input. When the voltage of the capacitor is dropped to 0 V, the input device will recharge the capacitor. A conventional input device uses a DC power supply and a serially connected variable resistor to charge the capacitor inside the game port. The resistance of the variable resistor controls the charging time of the capacitor. The variation over the variable resistor changes the time period detected by the detecting circuit of the game part and the time period is periodically transmitted to the computer as input signals.
Using variable resistors to generate input signals may cause the following three problems. First, a variable resistor is a passive element and its resistance varies in different environmental conditions. This may downgrade the accuracy and stability of the input signals. Second, friction in a variable resistor may cause mechanical damage to the variable resistor if it is frequently or forcefully used such as the variable resistors used in a joystick. Third, variable resistors must be constantly calibrated in order to improve its accuracy. This is quite inconvenient.